1. Field of the Invention
The present invention relates to scanning tunneling microscopes and, more particularly, to a scanning tunneling microscope for scanning large sample areas.
2. Description of the Prior Art
A scanning tunneling microscope scans a tip across the surface of a sample to measure its topography and this tip must be about three or four atoms away from the surface in order for the instrument to function. Only when the tip is this close to the surface does tunneling current flow between the sample and the tip. Because of this severe requirement that the tip be very close to the surface in order to operate, a scanning tunneling microscope must be very rigid. If the device is not rigid, the tip may strike the surface during a scan and destroy the sharpness of the tip. For measuring large samples this becomes a problem. A scanning tunneling microscope which is large enough to scan a large sample will not be sufficiently rigid and will suffer from unacceptably large thermal drifts between the components. Other similar microscopes, such as the atomic force microscope or scanning thermal probe microscope, use similar technology. I will refer to all these microscopes generally as "scanning probe microscopes."
My U.S. application Ser. No. 206,091, filed June 13, 1988, discloses a scanning tunneling microscope which rests on large samples so that the rigidity between the microscope scanner and the sample remains intact, but this and other prior art scanning tunneling microscopes are restricted in their scanning range to a limited portion of the sample surface which is about 100.times.100 microns. There is a need, however, when investigating large surfaces such as hard disc surfaces, for a system for moving the scanning tunneling microscope relative to the sample so that different portions of the sample can be measured.
U.S. Pat. No. 4,798,989 is concerned with the problem of a limited scanning area in scanning tunneling microscopes. The scanning tunneling microscope is incorporated in a sample chamber of a scanning electron microscope. A base is movable in an X-Y plane by a sample position adjusting device made up of a pair of perpendicular feed screw mechanisms, and a sample moving mechanism is mounted on the base and fixed in any desired position in the X-Y plane by changing piezoelectric elements. In such a device, the probe moving mechanism and the sample must be miniature enough to be accommodated in the sample chamber of a scanning electron microscope. Moreover, the scanning tunneling microscope is supported above the sample and does not rest on the sample with a resulting loss of rigidity.